In normal operating condition, U-I and P-I characteristic curves of a photovoltaic cell may vary with irradiation and temperature and present typical nonlinear characteristics. Because of low voltage and current output by a single photovoltaic module, a plurality of photovoltaic modules are usually connected in series and in parallel, and electric energy is converted afterwards in engineering. In this case, numerous factors, such as cell panels being blocked by clouds or other objects and inconsistency in temperatures and arrangement orientations of modules, may lead to power mismatch of a string of photovoltaic modules. MPPT (maximum power point tracking, i.e., holding a distributed power generation system in its maximum power output state) control mode is a commonly used means to solve this problem.
Traditional way for solving MPPT within a wide input range of a photovoltaic array is using string-type inverters in which boost modules and inverter modules are included. However, such solution have the disadvantages of high costs, small power per unit, complicated wiring due to an AC output parallel structure, and poor adaptability of a multi-parallel power grid. Moreover, in the current photovoltaic plant, increasing importance has been further attached to the protection of DC arc and inhibition of PID (Potential Induced Degradation, i.e., degradation of cells caused by bias voltage between modules and the frame) effect of a photovoltaic array with the disadvantage of high costs due to the addition of corresponding individual devices that are necessary to realize these functions.